1. Technical Field
This invention relates to positioning assemblies utilizing electrical linear motors and, more particularly, to assemblies that accelerate a movable tooling element approaching or exceeding one G.
2. Discussion of the Prior Art
Moving a heavy-duty cutting tool to sequential positions for carrying out a machining task has been conventionally carried out by use of three-axis positioners actuated and controlled by ball-screw drives (see U.S. Pat. Nos. 4,102,035 and 4,359,814). If electrical linear motors were to be substituted for such ball-screw drives, the disadvantageous slow wind-up characteristic of the ball-screw drive would be eliminated and unprecedented acceleration/deceleration would be possible.
Unfortunately, accuracy of positioning of the tooling would be diminished by the mere substitution, without uniquely controlling the force generated by the motors. Moreover, power control alone is insufficient to assure position preciseness if the assembly flexes or distorts variably in reaction to the thrust force of each motor. Such distortion is related not only to the natural flexing frequency of the supporting structure but also to the type of bearing contact with the movable elements.
Heretofore, table positioners using linear motors have employed air bearings (see U.S. Pat Nos. 4,392,642; 4,571,799; 4,985,651; and 4,761,876). Air bearings are made feasible by use of ultra-flat, inflexible granite surfaces and by limiting the movable element to a very light element such as needed for assembly or machining of light electronic components. Air bearings are undesirable because they require special support and guide surfaces that cannot be maintained in a heavy-duty, mass-machining environment for automotive components, and are undesirable because they are insufficiently dimensionally stiff when deployed to move large tooling at high accelerations.
Magnetic loading to increase stiffness of a linear motor assembly has been used in conjunction with sliding or roller bearings (see U.S. Pat. No. 4,505,464 and 4,985,651). Magnetic loading of bearings does little to enhance stiffness because it is imprecise and weak; magnetic loading is primarily suited to a use that assists in following more closely a guide track and therefore does little to promote stiffness of the linear motor assembly.